The main problem with manual video editing today is that it requires a huge amount of work from the user: the user must manually go over the entire input raw videos, mark starting and ending points for each portion of the video that he would like to select. Then, in order to get a pleasing edited video the user also needs to synchronize the selected clips with the soundtrack, and create graphical effects and transitions that fit the content of the video.
This process highly time consuming—it can take hours or even days. The resulting video clip, in many cases, doesn't have a professional look: The videos are not stable, the effects are amateurish and the synchronization with the soundtrack is not perfect.